Single call dial restoral, or backup, of leased multidrop data communications lines is traditionally carried out by using a two-wire "dial restoral" modem at the central, or control, site and one at the remote, or tributary, site for each tributary site which is "restored." The dial restoral modems at the control site must be digitally bridged to the leased line device, which is either a modem (for analog transmission) or a data service unit (DSU) for digital transmission.
Due to this digital bridging, the entire network, comprised of the leased line and all of the dial lines, must operate at the same speed, i.e., data bit rate. This causes significant system problems for the designer or user of the system. Since the dial restoral modems are limited in the speed at which they can operate (today that speed is about 14.4 kbps), any leased line which operates at a rate higher than that must be slowed down to the speed of the dial line.
For modems, this can be done by changing the speed of the control leased line modem. For DSUs, however, the situation is more difficult since the speed of the line is fixed by the service provider and cannot be changed by the user. Two solutions have traditionally been suggested for this situation. The first is to provide a multiplexer in both the control and remote DSUs which can be switched in and operated at the lower speed when going to dial restoral mode. Thus, even though the digital leased line operates at a speed higher than the dial modem, the speed of the data terminal equipment (DTE) served by the network is reduced to the speed of the dial restoral modem. The second method suggested is to initiate a dial restoral for every tributary connection, or "drop," whenever any one drop must be restored.
All of these solutions (for both analog and digital leased lines) can significantly reduce the performance of the overall line (for example, a 56 kbps line backed up at 0.6 kbps). The solutions known for the digital environment are also expensive as a result of their substantial equipment requirements.
Another problem arises when several drops must be restored. In this case, a subsequent restoral attempt may not be able to restore at the same speed that a previous restoral was accomplished due to impairments in the access line to that drop. There are two alternatives in this case. The first is to ignore this drop and not back it up. The second is to somehow communicate to all other devices on the "network" the necessity of falling back to the lower speed. Even if this can be accomplished, the performance of the network will be limited to the worse case connection speed.